I haven’t seen any evidence that the Mi 8 delivers decimeter-level precision — all accuracy plots I’ve seen have error in the range of several meters.

I haven’t used MIT App Inventor myself, but I imagine it probably doesn’t provide access to the raw pseudorange measurements. If that’s what you’re looking for, you can check out the source code of GPSTest to see how it listens to the APIs for pseudorange measurements — https://github.com/barbeau/gpstest.